The present invention relates to nonaqueous electrolyte secondary cells which comprise a can and a rolled-up electrode unit accommodated in the can and serving as a secondary cell element and which are adapted to deliver electric power generated by the electrode unit from a pair of electrode terminals provided on the can.
In recent years, attention has been directed to lithium secondary cells or batteries having a high energy density for use as power sources for portable electronic devices, electric motor vehicles, etc. Cylindrical lithium secondary cells of relatively large capacity, for example, for use in electric motor vehicles comprise, as shown in FIGS. 11 and 12, a cylindrical cell can 1 having a cylinder 11 and lids 12, 12 welded to the respective ends of the cylinder, and a rolled-up electrode unit 4 encased in the can 1. A pair of positive and negative electrode terminal assemblies 9, 9 are attached to the lids 12, 12, respectively. The two electrodes of the rolled-up electrode unit 4 are connected to the terminal assemblies 9, 9, whereby the electric power generated by the electrode unit 4 can be delivered to an external device from the pair of terminal assemblies 9, 9. Each lid 12 is provided with a gas vent valve 13.
As shown in FIG. 13, the rolled-up electrode unit 4 comprises a positive electrode 41 and a negative electrode 43 which are each in the form of a strip and which are rolled up into a spiral form with a striplike separator 42 interposed between the electrodes. The positive electrode 41 comprises a striplike current collector foil 45 in the form of aluminum foil and coated over opposite surfaces thereof with a positive electrode active substance 44 comprising a lithium containing composite oxide. The negative electrode 43 comprises a striplike current collector foil 47 in the form of copper foil and coated over opposite surfaces thereof with a negative electrode active substance 46 containing a carbon material. The separator 42 is impregnated with a nonaqueous electrolyte.
The positive electrode 41 and the negative electrode 43 are each superposed on the respective separators 42, as displaced from the separator widthwise thereof, and the assembly is rolled up into a spiral form, whereby the edge 48 of the current collector foil 45 of the positive electrode 41 is positioned as projected outward beyond the edge of the separator 42 at one of the axially opposite ends of the electrode unit 4, and the edge 48 of the current collector foil 47 of the negative electrode 43 is positioned as projected outward beyond the edge of the separator 42 at the other end of the unit 4. A current collecting plate 32 in the form of a disk is joined to each of the opposite ends of the electrode unit 4 by resistance welding and connected by a lead member 33 to the base end of the electrode terminal assembly 9 shown in FIG. 12.
The electrode terminal assembly 9 comprises an electrode terminal 91 extending through a hole in the lid 12 of the can 1 and mounted on the lid 12. The electrode terminal 91 has a flange 92 at its base end. An insulating packing 93 is fitted in the hole of the lid 12 for electrically insulating the electrode terminal 91 from the lid 12 and providing a seal therebetween. The electrode terminal 91 has a washer 94 fitted therearound from outside the lid 12, and a first nut 95 and a second nut 96 screwed thereon. The first nut 95 is tightened up to clamp the insulating packing 93 between the flange 92 of the terminal 91 and the washer 94 and thereby seal off the hole more effectively. The outer end of the lead member 33 is fixedly joined to the flange 92 of the terminal 91 by spot welding or ultrasonic welding.
The lithium secondary cell having the current collecting structure shown in FIG. 12 nevertheless has the problem that the edges 48, 48 of the current collector foils 45, 47 forming the positive electrode 41 and the negative electrode 43 of the rolled-up electrode unit 4 have a small area, which results in a small area of contact between each edge of the current collector foil and the corresponding current collecting plate 32, consequently increasing the internal resistance of the cell. Further when the outermost peripheral portion of the current collecting plate 32 is joined to the electrode edge positioned radially most outwardly of the electrode unit 4 by laser welding, the laser beam is likely to leak out from the collecting plate 32 to irradiate the electrode or separator, causing damage to the electrode or separator.
A cylindrical secondary cell of improved power characteristics has been proposed which, as seen in FIG. 17, comprises a positive electrode 81 having an uncoated portion which extends upward beyond a portion thereof coated with an active substance 84 and varies in width longitudinally of the electrode, and a negative electrode 82 having an uncoated portion which extends downward beyond a portion thereof coated with an active substance 85 and varies in width longitudinally of the electrode. The positive and negative electrodes 81, 82 are rolled up into a spiral form with a separator 83 interposed between the electrodes to obtain a rolled-up electrode unit 8 having conical projections 86 as seen in FIG. 18. The electrode unit 8 is encased in a cell can 1. Each of the electrode projections 86 is connected to an electrode terminal 90 by a current collecting lead 80 (JP-A No. 329398/1998).
Although improved to some extent in power characteristics, the secondary cell described requires the step of obliquely cutting an edge of each of the positive and negative electrodes 81, 82 as shown in FIG. 17. This not only makes the fabrication process complex but also presents difficulty in giving an accurately finished conical surface to the projection 86 of the rolled-up electrode unit 8 as shown in FIG. 18 by rolling up the assembly of the two electrodes, consequently entailing the problem of an impaired yield and variations in the properties of cells. Especially in the case of lithium secondary cells for use as power sources in electric motor vehicles, there is a need to reduce the internal resistance to the greatest possible extent so as to obtain a high capacity and a high power. Furthermore, a manufacturing cost reduction requires a current collecting structure of high productivity.
Accordingly, a nonaqueous electrolyte secondary cell having low resistance and excellent in productivity is proposed which has a current collecting plate 7 of the shape shown in FIG. 27 (JP-B No. 4102/1990). The collecting plate 7 has a central hole 74 and a lead portion 75 extending from the outer periphery thereof. The collecting plate 7 further has a plurality of ridges 72 V-shaped in cross section and extending radially from its center. As shown in FIG. 28, these ridges 72 are pressed against and weld to edge portions 48 of electrode of a rolled-up electrode unit 4.
With this cell, the ridges 72 of the collecting plate 7 bite in the edge portions 48 of electrode of the electrode unit 4. The collecting plate is therefore in contact with the edge 48 of the electrode over a greater area than the conventional collecting plate which is in the form of a flat plate. This results in an increase in the quantity of current collected to afford an increased cell power.
However, since the ridges of the collecting plate have a V-shaped cross section with an acute angle, the area of contact of the ridges with the edge of the current collector foil is not sufficiently great. Accordingly, the collecting plate not only has great contact resistance at the weld but is also poor in the state of contact at the portions other than the weld. Thus, the structure described has the problem of low current collecting performance. Moreover, the junction between the V-shaped ridge and the edge of the current collector foil to be irradiated with a laser beam makes an acute angle with the direction of projection of the beam, so that the laser beam fails to act effectively on the junction for welding and is likely to produce a faulty weld joint.
Further for the nonaqueous electrolyte secondary cell to give an improved power, it is effective to reduce the electric resistance of the path through which the electric power produced by the rolled-up electrode unit 4 is delivered to the outside, i.e., the internal resistance, whereas the current collecting plate 7 shown in FIG. 27 is greater in the average distance over which the current collected by the plate 7 flows before flowing into the lead portion 75 because the lead portion 75 extends from the outer periphery of the plate 7. For this reason, the secondary cell incorporating the collecting plate 7 still has great internal resistance.
A first object of the present invention is to provide a nonaqueous electrolyte secondary cell having a current collecting structure wherein even when the current collectors forming the electrode unit are very thin, the current collector edge is held in satisfactory contact with a current collecting plate to ensure high current collecting performance and which is excellent also in productivity.
A second object of the present invention is to provide a cylindrical secondary cell of the tabless type wherein a current collecting plate can be welded to the end of a rolled-up electrode unit without the likelihood of causing damage to the electrode or the separator and which can be fabricated by a simple process, the cell further exhibiting excellent power characteristics.
A third object of the present invention is to provide a nonaqueous electrolyte secondary cell which has a current collecting structure comprising a current collecting plate and which is smaller than conventionally in internal resistance.
Construction for Fulfilling the First Object
The present invention provides a nonaqueous electrolyte secondary cell wherein a current collector forming a positive electrode 41 or a negative electrode 43 has an edge 48 projecting from at least one of opposite ends of a rolled-up electrode unit 4, the current collector edge 48 being covered with a current collecting plate 5.
The current collecting plate 5 has a plurality of circular-arc protrusions 52 projecting in a circular-arc cross section toward the current collector edge 48 and a plurality of slit pieces cut to a raised form toward the current collector edge 48, the circular-arc protrusions 52 being welded to the current collector edge 48 while biting into the current collector edge 48 along with the slit pieces 53. The current collecting plate 5 is connected to one of a pair of electrode terminal portions.
With the nonaqueous electrolyte secondary cell of the present invention, the current collecting plate 5 is pressed against the current collector edge 48 of the rolled-up electrode unit 4, whereby each circular-arc protrusion 52 is caused to bite into the current collector edge 48, forming on the current collector edge 48 a cylindrical junction shaped in conformity with the shape of surface of the protrusion 52. The junction has a large area than when the protrusion is V-shaped in cross section. The slit pieces 53 also bite deep into the current collector edge 48, resulting in a satisfactory state of contact between the collecting plate 5 and the current collector edge 48 at regions other than the welds.
Accordingly, the collecting plate 5 can be joined to the current collector edge 48 over increased areas of contact by irradiating the junctions of the circular-arc protrusions 52 and the current collector edge 48 with a 15 laser beam or electron beam and thereby welding the collecting plate 5 to the current collector edge 48. This results in reduced contact resistance and high current collecting performance.
The junction of each protrusion 52 of the current collecting plate 5 and the current collector edge 48 has its central portion positioned at an angle of 90 degrees or an angle approximate thereto with the direction of projection of the beam. This permits the laser beam or electron beam to act effectively on the junction for welding, whereby a high weld strength is available due to an increased joint area.
Stated more specifically, the current collecting plate 5 comprises a disklike body 51 having the circular-arc protrusions 52 and the slit pieces 53 formed radially and opposed to the current collector edge 48, and a striplike lead portion 55 extending from an edge portion of the disklike body 51 and joined at an outer end thereof to the electrode terminal portion. In this specific construction, the current produced by the rolled-up electrode unit 4 is collected by the collecting plate 5 and flows to the electrode terminal portion via the lead portion 55.
Stated more specifically, each of the slit pieces 53 is in contact with the current collector edge 48 over a length at least 0.5 times the radius of the current collecting plate 5. This provides a sufficiently wide area of contact between the current collecting plate 5 and the current collector edge 48 for high current collecting performance.
Each of the slit pieces 53 projects toward the current collector edge 48 over a length at least 1.0 times to not greater than 1.5 times the length of projection of the circular-arc protrusion 52. This enables the protrusion 52 to come into contact with the current collector edge 48 over a wide area, while the slit piece 53 bites into the current collector edge 48 to a sufficient depth.
The current collecting plate 5 can be made from Cu, Al, Ni, SUS, Ti or an alloy of such metals. The cell thus provided is excellent in corrosion resistance to the nonaqueous electrolyte and in electric conductivity.
Even when the rolled-up electrode unit of the cell of the invention comprises a very thin current collector, the current collector edge can be held in contact with the collecting plate over increased areas, while the cell can be provided with high productivity.
Construction for Fulfilling the Second Object
The present invention provides a cylindrical secondary cell comprising a positive electrode 41 and a negative electrode 43 each in the form of a strip and rolled up into a spiral form with a separator 42 interposed between the electrodes and impregnated with a nonaqueous electrolyte to obtain a rolled-up electrode unit 4, and a cylindrical cell can 1 having the rolled-up electrode unit 4 accommodated therein, the cell being adapted to deliver electric power generated by the rolled-up electrode unit 4 to the outside via a pair of electrode terminal portions.
The positive electrode 41 and the negative electrode 43 each comprise a striplike current collector and an active substance coating the current collector, each of the electrodes having a portion coated with the active substance and extending longitudinally of the current collector, and an uncoated portion not coated with the active substance and formed along an edge of the current collector.
The uncoated portion projects from at least one of axially opposite ends of the rolled-up electrode unit 4 to provide a cylindrical projection 40, which is covered with a current collecting plate 6 made of a metal. The current collecting plate 6 comprises a top plate 61 in contact with an end face of the cylindrical projection 40 and a skirt portion 62 in contact with at least a portion of an outer peripheral surface of the cylindrical projection 40. The current collecting plate 6 is connected to one of the electrode terminal portions by a lead member 63.
With the cylindrical secondary cell of the invention described, the end face of the cylindrical projection 40 of the rolled-up electrode unit 4 and the inner surface of the top plate 61 of the current collecting plate 6 are in contact with each other, and the outer peripheral surface of the cylindrical projection 40 and the inner peripheral surface of the skirt portion 62 of the collecting plate 6 are also in contact with each other, with the result that the contact resistance between the electrode of the unit 4 and the collecting plate 6 is low, consequently giving reduced internal resistance to the cell and permitting the cell to exhibit high power characteristics.
In joining the outermost peripheral portion of top plate 61 of the collecting plate 6 to the portion of electrode edge positioned at the outermost peripheral portion of the electrode unit 4 in the step of laser welding of the collecting plate 6 as fitted over the cylindrical projection 40 of the electrode unit 4, the outer peripheral surface of the cylindrical projection 40 is covered with the skirt portion 62 of the collecting plate 6. The skirt portion therefore obviates the likelihood that the electrode or separator will be exposed directly to the laser beam, preventing damage to the electrode or separator.
Furthermore, the positive electrode 41 and the negative electrode 43 forming the electrode unit 4 need only to be made each in the form of a strip having a specified width. This simplifies the fabrication process, further making it possible to give the cylindrical projection 40 of the unit 4 with an accurately finished cylindrical surface and consequently eliminating a reduction in the yield and variations in the cell performance.
Stated specifically, the top plate 61 and the skirt portion 62 of the current collecting plate 6 are joined respectively to the end face and the outer peripheral surface of the cylindrical projection 40 of the rolled-up electrode unit 4 by laser welding. This fully reduces the contact resistance between the electrode unit 4 and the collecting plate 6.
Thus, the current collecting plate can be welded to the rolled-up electrode unit without the likelihood of causing damage to the electrode or separator, so that the cylindrical secondary cell of the invention is easy to fabricate. Moreover, the reduced internal resistance of the cell assures outstanding power characteristics.
Construction for Fulfilling the Third Object
The present invention further provides a nonaqueous electrolyte secondary cell comprising an electrode unit encased in a cell can, the electrode unit comprising as superposed in layers a pair of positive and negative electrodes and a separator interposed between the electrodes and impregnated with a nonaqueous electrolyte, the cell being adapted to deliver electric power generated by the electrode unit to the outside via a pair of electrode terminal portions provided respectively at opposite ends of the cell can, the nonaqueous electrolyte secondary cell being characterized in that an edge of a current collector forming the electrode projects from at least one of opposite ends of the electrode unit, a current collecting plate being joined to the edge and having a male screw projecting from a surface of the plate toward the electrode terminal portion, the male screw being in screw-thread engagement with an internally threaded portion formed in the electrode terminal portion.
With the nonaqueous electrolyte secondary cell of the invention described, the male screw provided on the surface of the current collecting plate is driven directly in the electrode terminal portion, forming the shortest current path between the current collecting plate and the terminal portion and consequently reducing the internal resistance of the cell.
Stated more specifically, the male screw is integral with the current collecting plate. This structure has no joint between the collecting plate and the male screw and no contact resistance, consequently reducing the internal resistance of the cell.
Alternatively, a base plate is provided on the surface of the current collecting plate centrally thereof, and the male screw is provided on a surface of the base plate. With this structure, the base plate provided with the male screw is a member separate from the collecting plate, and can therefore be made from a material of low resistance different from the material of the collecting plate. With the male screw positioned centrally of the collecting plate, the average distance the current collected by the collecting plate flows before reaching the male screw is short, consequently reducing the internal resistance of the cell.
Further alternatively, the opposite ends of the electrode unit have edges of current collectors forming the respective electrodes projecting therefrom, and the current collector edges have respective current collecting plates joined thereto, the male screw projecting from one of the current collecting plates, the other current collecting plate being provided with a connecting member projecting therefrom and having elasticity so as to move toward or away from the electrode terminal portion. With this specific construction, the connecting member having elasticity is provided between the electrode terminal portion and the current collecting plate at one end of the electrode unit, so that the errors involved in assembling the electrode unit or the cell can is absorbable by the elastic deformation of the connecting member. This eliminates the need for strict dimensional management, leading to an improved cell production efficiency.
Thus, the nonaqueous electrolyte secondary cell provided by the invention has lower internal resistance than conventionally.